As one means to correct device errors produced by the limited manufacturing accuracy of integrated circuits, e.g., analog integrated circuits, an adjusting method of on-chip trimming named "Zener Zap Trimming" has been known. This method uses a non-volatile ON/OFF switch which is, so to speak, writable only one time using a phenomenon that a zener diode is broken (zapped) and is permanently short-circuited if an electric pulse above a certain energy is applied to the zener diode in the reverse direction.
By prearranging some pieces of such zener diodes in a circuit, the zener diodes are used in the OFF (open-circuit) state under a bias condition so that the zener diodes do not close in the forward direction and no voltage above the zener voltage is added in the reverse direction if the zener diodes are not zapped. When making the trimming, while checking circuit errors, a designated point is decided and zapped and a total error is reduced by turning a combination of some proper zener diodes ON (short-circuited).
The most basic example of application of a conventional zener zap trimming is a resistor trimming. For this trimming, a resistor network is constructed by a plurality of resistors which are subject to the trimming and zener diodes are connected to these resistors in series or in parallel, and pads are arranged at both ends of each zener diode. A resistance error is corrected by deciding a proper combination of ON/OFF state of the zener diodes paired with the resistors and by zapping the resistor network.
Further, instead of the direct trimming of a resistor network, there is also an indirect trimming method to adjust a circuit subject to the trimming by a D/A output by constructing a network using a simple D/A conversion circuit and data to be input to this circuit can be set by the zener zap trimming. For instance, when this trimming circuit is used for an adjustable filter circuit, an oscillator or for time constant adjustment, it is possible to adjust at a common location several circuits whose time constant fluctuation is a problem and therefore, this trimming circuit is suited to such uses.
FIG. 1 shows a conventional circuit having a current output type D/A output as one example of the D/A conversion type trimming. A voltage-current converter is constructed by an operational amplifier B80 and a transistor Q80, reference voltage VB is constantly applied between the emitter of the transistor Q80 and GND and current converted according to a resistance between the emitter of the transistor Q80 and GND is output to a current output terminal T80 through the collector of the transistor Q80.
Therefore, by deciding the ON/OFF for connecting resistors R80b through R80d to GND by zener diodes Z81 through Z83, output current value obtainable at the terminal T80 can be varied. In this circuit example, an output current Iout can be varied in 8 kinds as follows by the combination of ON/OFF of zener diodes Z81 through Z83: EQU Iout=nVb/8R, (n=8, 9, 19, 11, 12, 13, 14, 15) (1)
Pads P81 through P83 are for receiving zapping pulses and by applying voltage pulse to the pads corresponding to the zener diodes Z81 through Z83 from the outside, it becomes possible to short (ON) a designated point only.
Shown in FIG. 2 is an example of the zener diodes zapping circuit shown in FIG. 1. In this example, the zener diodes Z81 through Z83 have been substituted by switching transistors Q91 through Q93 and these switching transistors are controlled by the zener diodes Z91 through Z93.
When the zener diodes Z91 through Z93 have not been zapped, the switching transistors Q91 through Q93 are biased by resistors R91 through R93 and R94 through R96, and the switches are in the ON state. When the zener diodes have been zapped, bias current from Vcc is shorted to GND in the middle and therefore, no bias current is supplied to the switching transistors and the switches are in the OFF (open-circuit) state.
Thus, as in the example shown in FIG. 1, it is possible to vary the output current of the D/A conversion circuit by selectively zapping the zener diodes. However, the output current is reversed one time by the switching transistors Q91 through Q93, and the relationship between the zapping and the switch ON/OFF becomes contrary to the example in FIG. 1. This circuit is superior to the circuit shown in FIG. 1 in that current flowing to zener diodes in the short-circuited state after completing the zapping is easily restricted and the other point is that residual resistance and its fluctuation, even when large, scarcely affect D/A conversion output, and this circuit has a higher using value than the circuit shown in FIG. 1.
In the conventional circuit examples shown above, at least one zapping pad is needed for every zener diode. In the examples in FIGS. 1 and 2, as the anode terminal of zener diode is of GND type and a GND pad can be used when zapping, only one pad is required for zapping per zener diode. However, two pads may be required for each zener diode depending on a type of circuits. If the number of trimming points by zapping of zener diodes is increased or if the number of trimmings is increased in order to increase the number of trimming steps, the number of pads must be increased proportionally and therefore, economy will become remarkably worse. For instance, if there are an average five trimming points for 4 bits, more than 20 pads must be specially arranged for the zapping.
In such trimming according to the conventional zener diode zapping system, the trimming can be performed only in the wafer before assembling (before sealing in a package) an integrated circuit. This is so because in order to perform the trimming by zapping after assembling an IC, it is necessary to put all the zapping pads out as IC pins and as a result, the number of IC pins increases by at least the product of the number of trimming points and the number of average trimming bits. Such an arrangement is not practical.
However, a demand for trimming of IC in the state sealed in a package after assembled is strongly demanded mainly for the following four reasons.
The first reason is that if the trimming was performed in the wafer state, there is such a defect that the correction made by the trimming may shift and trimming accuracy cannot be promoted because of the effect of stress applied to the mount during the assembling and the piezoelectric effect of stress between an IC chip and resin when sealing in resin mold, and thus it is necessary to perform the trimming requiring a high accuracy in the state close to the final product form after assembled.
The second reason is that the ICs may be rejected as being faulty after a die-sorter test. Therefore, any trimming of ICs prior to such test may be unnecessary. The trimming by the zapping requires labor and cost and therefore, in particular, if a yield of good products in the test after assembled is worse, it will be more economically advantageous to perform the trimming after the test after assembly than to perform the trimming in the state of water.
The third reason is that when the trimming is performed while checking signals by putting ICs in The operating state, the ICs cannot be accurately checked in the wafer state as it is complicated. The trimming of ICs in the wafer state by the zapping is sufficient for the trimming of simple resistance value. Actually, however, the trimming is executed while checking a frequency characteristic and an amplitude characteristic in a considerably large scale circuit in many cases. In these cases, it is necessary to carry out the precise measurement and a high level of discrimination by an AC test instead of a simple DC test and a general purpose equipment of die sorter which is carried out on ICs in the wafer state is insufficient in many cases. In this point, the trimming after assembly allows a large degree of freedom from the viewpoint of equipment and operation.
The fourth reason is that there is such a case where equipment makers who are users of ICs perform the trimming of overall characteristic including fluctuation of a set not only the self-optimizing type trimming of simple ICs. In this case, manufacturing of products which allows the trimming by the zapping will become an absolute condition required for the zener zap trimming of integrated circuits.
As to the conventional trimming in ICs, it is strongly demanded to make the trimming on ICs in a form close to the final products after assembled from the viewpoints of in assuring trimming accuracy, economy, the degree of freedom for monitoring and measuring, trimming by use makers, etc. However, there was such a problem that in order to perform the trimming after assembling ICs according to the conventional zener zap trimming system, a large number of IC pins dedicated for the zener zapping are needed which is extremely uneconomical and not practical.